Why did my IK bone shrink?
You've changed its scale, in Pose mode you need to select it and alt S to reset its scale, you can also reset its rotation (not sure why you put the target in front of the foot and not behind, though. Also, I'd make the target parent of the foot but there are a lot of rigging methods so I don't know).



Now, if you want to delete scale and rotation for the whole animation of this bone, select the bone, in the bottom menu bar of the Dopesheet press the arrow button called "Only Include Channels Relating to Selected Object and Data" so that it only displays the keyframes of the selected bone, lock the tracks you don't want to delete (in your case the 3 Location tracks), select all the keyframes (A) and press X > Delete Keyframes.



How did this happen? You must have accidentally scaled this bone at the beginning of the walk, and as you've chosen to automatically save the Whole Character pose when you press i, it automatically recorded the scale setup.